The overall goal of this project is to study the early postnatal development of neuronal organization in structures belonging to the visual and basal ganglia systems of the monkey brain. Such goal will be accomplished at the light and electron microscopic levels with special emphasis on quantitative morphology. More specifically, structures to be investigated will be the lateral geniculate nucleus pars dorsalis and pars ventralis, the neostriatum and the pallidum of rhesus monkeys at 0 (newborn), 1, 2, 4 and 8 weeks of age. Specimens from other components of the systems will also be collected for future studies. Light microscopy methods will include Nissl and Golgi series. Electron microscopy will make use of both single and serial sections. Quantitative methods will be based on the principles of stereology and will include determinations of the number, volume and density of various profiles; the number, size, density and maturational stage of various synaptic contacts; and the packing density of synaptic vesicles. The changes occurring over time in the first weeks of the life of a primate will help to understand the neuronal organization of the fully grown subject and will give a direct comparison of the maturational rate of a diencephalic sensory system as compared to a more complex telencephalic system traditionally involved in motor function. This information can provide insight into the morphologic basis for the development of visual behavior as well as on the possible nature of certain abnormalities in motor function acquired in the perinatal period of man.